System and method for rapid charging of a battery

ABSTRACT

Method and apparatus for high-rate charging of batteries with sealed cells, whereby through supplied charging current the efficiency the single battery cell is actively influenced such that the charging time and temperature rise of the cell is minimized such that in a first step the battery is discharged to a voltage U B  slightly higher than a first reference voltage (U o ) whereafter in a first recharging step the supplied charging current I B  is controlled progressively increasing according to the function I B  =k(U B  -U o ) where k is an adjusted constant, until the pole voltage has reached a second reference voltage U B  -u where u is maximum batter voltage and in a second charging step the charging current I B  is controlled through voltage feedback such that U B  ≈u whereby the amount of the supplied charging current is determine by the charging state of the battery cells. The first and second charging step can also be repeated with a fixed time interval with a free running time interval without charging current a number of cycles until a state of full charge is reached.

The present invention refers to a method and a device to supply electricenergy to a rechargeable battery in such a way that a substantialreduction of the charging time is obtained compared to previously knownmethods and devices. The method according to the invention is mainlyintended for Ni-Cd batteries.

TECHNICAL PROBLEM

A conventional method of charging especially sealed Ni-Cd batteries issupplying a constant current during a certain time which in itself is asimple and reliable method. The manufacturers of this type of batteriesrecommend a charging current of 0.1 times the capacity of the cell (0.1C Amperes)*, which gives more than 10 hours of charging. Moreover inorder to obtain a fully charged battery at this low charging current anextra time of 4 hours is required for full charge. Thus the totalcharging time with conventional methods is about 14 hours. The value ofthe charging current is defined by the quantity of oxygen that can beburned at the Cd-electrode in a fully charged state of the cell. Inorder to explain the function of a Ni-Cd - cell, accompanying FIGS. 1A,1B and 1C show different measured charging characteristics constantcharging current in which particularly

FIG. 1A shows cell voltage as a function of input charge at a certaincharging current (0.2 C) at different temperatures.

FIG. 1B shows charging acceptance or charging efficiency for differentconstant charging currents at room temperature

FIG. 1C shows charging acceptance or charging efficiency at roomtemperature being the ambient temperature at different cell temperatureswith a normal charging rate of 0.1 C.

The problem being fundamental to the invention is that a user must haveaccess to spare batteries for continuous operation of a battery operatedsystem if the decharging time is shorter than charging time. A chargingtime of 14 hours and a decharging of 1h demands 14 batteries forcontinuous operation of the system.

State of the art

Through the U.S. Pat. No. 4,246,529 a battery charger is known in whichis used integrator means (4) an current switching means (8) incombination with a control circuit (7) whereby the control circuitreceives a synchronizing signal at the start of each cycle as determinedby a cyclical supply or pulse generator as well as information about thestate of the integrator means which is compared with the predeterminedaverage current. This information determines the required controlsignals for the integrator means and current switch means so as tomaintain the value of average charging current substantially constantthroughout the charging time. Current sense means (5) are provided tosense the current flow through the battery and supply the integrator (4)with said current.

In the above u-processor controlled battery charger for lead acidbatteries the energy is pulsed with relatively short pulses into thebattery and the average current is measured which can be expressed as

    Iav=1/π.sub.01 ∫.sup.02 (V.sub.c -V.sub.b)/R.sub.3 dθ

where V_(c) =charging voltage, V_(b) =battery pole voltage R₃ =currentsense resistance and θ=current angle In this system the chargingefficiency of the battery is not considered for different chargingcurrents. The equation above does not show the progressively increasinginitial course of charging current which is produced by the systemaccording to the present invention.

Through U.S. Pat. No. 3,987,353 is known a battery charging controlsystem in which the charging current is switched on and off at repeatedintervals, whereby the open battery voltage during the "off" periods isused to generate a control signal to vary the magnitude of the chargingcurrent during "on" periods. The control signal is derived throughextracting the resultant internal voltage drop (I. R Drop) at thebattery connections when the charging current is switched off andthereafter differentiate the rate of decay of the battery voltage bymeans of the differentiator (30). A voltage is then derived from thedifferential signal and is used in one of three ways to control themagnitude of the charging current in order to progressively reduce itsmagnitude once gas generation has been detected as a result of asignificant differential occurring.

In the above related control system for battery chargers which operateswith pulses divided from the line frequency and evidently intended forlead acid batteries, the magnitude of charging current is modified as afunction of the charging state of the battery which has been derivedfrom the "gas drop". The charging characteristics is related to theefficiency but is not directed to high-rate charging.

OBJECTS OF THE INVENTION

A first object of the invention is to minimize the charging time as faras possible by making use of the intrinsic properties of the Ni-Cd cell.A second object of the invention is to accomplish as high efficiency aspossible at indicated charging conditions.

TECHNICAL SOLUTION

The problem is solved according to the present invention by using aspecific property of the battery cell in such a way that, the largercharging current input in the cell, the greater charging acceptance orcharging efficiency is obtained, see diagram A in FIG. 1. The chargingtime is shortened, partly because of the higher charging current andpartly because of the higher acceptance or efficiency. This chargingmethod according to the invention moreover has the favorable effectthat, when the acceptance is enhanced, less of the input energy istransformed into heat, which acts to restrict the temperature rise inthe cells.

As can be seen from the diagram in FIG. 1C the temperature of the cellshould be kept as low as possible for high acceptance or efficiency tobe maintained. The basic idea in the present invention is that theintrinsic properties are used actively in such a way that highestpossible acceptance for charging is obtained, which minimizes thecharging time. The upper limit of the charging current depends onphysical and chemical reasons such as the current density at the and therecombination of oxygen at the cadmium electrode. When verifying themethod according to the invention a maximum current of about 2.5* CAmperes has been used.

The charging method according to the present invention gives rise to aprogressive process, since the charging current increases as the inputelectrochemical charge increases. The increase in current is controlledby the voltage of the battery, which according to the diagram in FIG. 1Aincreases with input charge.

According to the invention the voltage level of the battery is used acoarse indication of full charge and transformed to a signal which isused for switching to charge with constant voltage.

Since the acceptance is not complete or efficiency of 100% cannot beobtained, a certain heating of the cells happens during charging. As canbe read from the diagram in FIG. 1A the voltage hunch of the curvedecreases with increasing cell temperature and with this the possibilityto positively detect an appropriate switching level (at constantcharging current). A progressively increasing charging current enhancesthe voltage hunch which is thus favorable.

Before start of charging the battery is discharged and relatively lowinitial current is chosen (about 0.5 C Amps) in order to avoid stress onthe cells.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to accompanying drawings a first and a second embodimentof a battery charging circuit which perform the method of chargingaccording to the present invention will be described.

FIG. 1A, 1B and 1C are diagrams which show different measured chargingcharacteristics at constant charging current;

FIG. 2 is a block diagram over a charging circuit according to a firstembodiment of the invention;

FIGS. 3A and 3B are time diagrams over different charging processes ofwhich FIG. 3A shows charging at room temperature and FIG. 3B showscharging at -20° C.;

FIG. 4 is block diagram over a microprocessor based system according toa second embodiment of the invention for rapid charging of batterycassettes in which is used the charging method according to theinvention.

With reference to FIG. 2 a Ni-Cd battery with the pole voltage U_(B) viaa control transistor 20 and current limiting resistor 41, which is alsoused as an integration resistor, connected to a DC voltage source 4. Acontrol circuit 2 is connected to the base of the control transistor 20and on one hand contains an amplifier 200 and on the other voltagecomparators 210 and 220. The comparator 210 has on one of the inputsmaximum battery voltage u as a reference and on the other U_(B). Thecomparator 220 on one of the inputs has a start voltage U_(O) as areference and on the other input U_(B). The level shift signals from thecomparators 210 and 220 are supplied to a control unit 3 which containsmemory and timing circuits so that charging can be supplied to thebattery during certain time intervals and absorbed energy measuredduring the whole charging process.

To form a control circuit to control the charging current the controlcircuit 2 is by way of a switch connected to summing points 22 and 23,of which the summation point 22 forms U_(B) -U_(O) and 23 forms u-U_(B)where u and U_(O) are reference voltages as above. U_(B) is supplied tothe control circuit 2 and the summation points 22, 23 by way of themeasure or sense line 112.

With reference to the diagrams in FIGS. 3A and 3B charging processes aredescribed, which are o with a device according to the above.

Before a charging process is started, the battery is discharged in orderto obtain a correct reference level for charging (with reference levelis referred to as ≈0 available charge) and supplied charge to bemeasured during a charging process. The discharge occurs by the controlcircuit activating the switch 21. The discharge time is defined bycomparing U_(B) with U_(O).

After a time of reactivation of the battery cells a new comparison withU_(O) is carried out in the comparator 220 to examine if the startingcondition U_(b) >U_(O) is satisfied.

When the starting condition is positive the switch 21 by way of thecontrol circuit 3 is set in position B which gives a charging current of

    I.sub.B =k(U.sub.B -U.sub.O)

In the initial moment I_(B) is small, since U_(B) is low. As the batterysuccessively accumulates charge, the pole voltage of the battery U_(B)increases. This voltage controls the charging current following thelinear function above, which gives an increasing voltage (positivefeedback). The process continues until U_(B) reaches the referencevoltage u. The comparator 210 thereby gives a level shift signal to thecontrol unit 3, which shifts the switch 21 to position A. This firstaccelerated charging phase takes the time T'.

With the switch in position A a charging current I_(B) is obtained whichgives the battery voltage

    U.sub.B ≈u

The system is now in a negative mode feedback and the pole voltage keptconstant. As the charge of the battery increases, the charging currentstarts to decay. After a total charging time T'+T", which has beencalculated such that a battery which was from the beginning dischargedhas become fully charged the charging current is switched off,alternatively connected to trickle charging by way of the input 203 onthe control circuit 2. By measuring supplied charge Q_(tot) by means ofan integrator built into the monitoring circuit and interrupt thecharging when the battery has absorbed a certain amount of chargeQ_(max) corresponding to a fully charged battery, the precision in thetime control is enhanced and the risk for overcharging is eliminated.

At low temperatures, eg. -20° C. the voltage u is rapidly reached in thefirst charging phase which extends the charging time. By cyclicallyrepeating the processes with the switch 21 in position A during the timeT' and in position B during the time T" with a pause T_(pause) see FIG.3B, an efficient charging in accomplished even at low temperatures. Thecharging current is integrated a number of cycles until a state of fullcharge has occurred.

In order to further enhance the efficiency and correspondingly shortenthe charging time at rapid charging of a cold battery the followingprocedure is carried out:

A "cold" battery is cycled a number of periods where every periodcontains a limited charging--discharging. By this the battery isprepared chemically for a rapid charging. By reading the current in thefirst charging process at the voltage u and compare this current with aminimum value of I_(B) an indication of "cold" battery is obtained andthe cycling mode of above is activated.

With reference to FIG. 4 in the following a rapid charging system forbattery cassettes comprising the charging circuit according to FIG. 2.

In the system below stated functional features are included:

Automatic control of discharging--charging of several battery cassettessimultaneously by means of a microprocessor which replaces the controlcircuit 2 and the control unit 3 in FIG. 2;

Automatic adjustment to correct battery type by every type of batterycassette having a unique jumper combination in the adapter;

In the control of discharging--charging process functions are includedsuch as control of charging current, measuring of battery voltage,measuring of supplied charge and measuring of total charging time;

Charging currents, charging times etc for different battery types arecoded in the program memory of the processor (PROM);

User interface which describes the charging process in the form of alight emitting diode display on which status is shown separately for all4 batteries and a display which indicates if the A- and/or B- cassetteis connected.

With reference to FIG. 4 the electronics system comprises a line blockAC/DC and a separate DC converter DC/DC to supply the electronic blocks.Incoming power 220 V AC or 24 V DC is switched on and off by way of theswitch S1. In the CPU- block CPU is included an 8 -bits micro-controller(Intel 80C31) which is provided with a program memory 32k EPROM, a datamemory 8k SRAM, watchdog--timer circuit and a 8-bits A/D converter. TheCPU- block controls and supervises the function of the battery chargerand communicates via a bus line with the display block DI and the analogblock AN. The analog block contains 4 units of 4-1 channel analogmultiplexers with address decoders for measuring channels and choice ofcharging mode and a double multiplying 8 bits D/A converter for controlof on one hand charging current directly from the CpU block and on theother control of the amplification G_(i) in the current control loop.The analog block further communicates further with the currentcontrol/measuring block CCO and battery cassettes A and B. The analogblock is interfaced towards the battery cassettes for control andmeasurement of charging current, receiving of temperature status T_(B),measuring of pole voltage U_(B), receiving of battery code CODE,generation of trickle charging and control of FET-switches SW1 and SW2to connect charging current and a discharging load. The charging isstarted and stopped by means of the nonlocking switch S2.

The display block DI operates a number of light emitting diodeindicators which for example through flashing indication shoe that workin the form of charging is carried out in a battery cassette A,B while alighted indicator shows that a work is completed which means that thebatteries in a cassette are fully charged. In order to illustrate thefunction of the battery charging system according to FIG. 4 an activitydiagram covering a charging process is given below.

    ______________________________________                                        Phase:                                                                              Activity:                                                               ______________________________________                                         1.   The indicator "CHARGING" is activated to mark that                            charging is going on;                                                    2.   The timer is set to zero to measure total charging time;                 3.   Accumulated charge, Q.sub.tot is set to zero                             4.   The gain in the current control loop is set by means of                       the gain factor G.sub.i ;                                                5.   Current control is activated                                             6.   Charge measuring is started;                                             7.   A current generator is connected to the battery;                         8.   The timer is set to zero for measuring of time in                             current control mode                                                     9.   If U.sub.B < U.sub.max after the time T.sub.icp the                           charging current is switched off and the indicator                            "ERROR " is alighted;                                                   10.   Shift to voltage control when U.sub.B = U.sub.max;                      11.   The timer is set to zero for measuring of time in voltage                     control mode;                                                           12.   After the time T.sub.vcp the charging current I.sub.B is                      measured;                                                                     If I.sub.B < I.sub.low the battery should be discharged T.sub.dis             or down to the voltage U.sub.dis. During discharge the remov-                 ed charge is calculated which is subtracted from Q.sub.tot              13.   If Q.sub.tot < Q.sub.max the charging current is switched off                 during the idle period T.sub.pause whereupon the charging cur-                rent is switched on and the charging continues from                           phase 5.                                                                14.   If Q.sub.tot < Q.sub.max after a total charging time > T.sub.max              the charging is interrupted and the indicator "ERROR" is alighted             to                                                                            indicate that the battery does not take charge;                         15.   When Q.sub.tot >= Q.sub.max the battery is fully charged and                  the charging current is interrupted. The indicator                            "CHARGING" is alighted.                                                 ______________________________________                                        Abbreviations:                                                                G.sub.i = Amplification factor to DAC in the current control loop             T.sub.icp = maximum time for current control phase                            T.sub.vcp = maximum time for voltage control phase                            T.sub.dis = maximum time for discharge in the charging cycle                  I.sub.low =  minimum current for discharge in the charging cycle              U.sub.dis = pole voltage level where discharge should be interrupted          U.sub.max = u = pole voltage level for transition to voltage control          Q.sub.max = Amount of charge which shall be supplied to the battery           to obtain full charge                                                     

I claim:
 1. Method for high-rate charging of batteries with sealedcells, whereby through supplied charging current the efficiency thesingle battery cell is actively influenced such that the charging timeand temperature rise of the cell is minimized characterized in that in afirst step the battery is discharged to a voltage U_(B) slightly higherthan a first reference voltage (U_(O)) whereafter in a first rechargingstep the supplied charging current I_(B) is controlled progressivelyincreasing according to the function

    I.sub.B =k(U.sub.B -U.sub.O)

where k is an adjusted constant, until the pole voltage has reached asecond reference voltage

    U.sub.B =u

where u is maximum battery voltage.
 2. Method according to claim 1characterized in that in a second recharging step the charging currentI_(B) is controlled through voltage feedback such that

    U.sub.B ≈u

whereby the amount of the supplied charging current is determined by thecharging state of the battery cells.
 3. Method according to claim 1characterized in that supplied charge is measured and that the first andsecond recharging step is followed by a certain time intervaldisconnected without charging current (T_(pause)) contained in acharging cycle and carrying out a number of cycles until a state of fullcharge corresponding to a certain amount of charge (Q_(max)) is reached.4. Apparatus for high-rate charging of batteries with sealed cellswhereby through supplied charging current the efficiency of the signalbattery cell is actively influenced such that the charging time andtemperature rise of the cell is minimized, said apparatus comprising:acontrol means in series with a DC current source, said control meansincludinga control electrode, a measuring conductor, summing circuit,and a control circuit connected to the control electrode whereby thecontrol circuit comprises a control amplifier and voltage comparatorswhich control circuit controls the charging current in response tosignals from the summing circuits which form respectively first andsecond differential control voltages (U_(B) -U_(o)), (u-U_(B)) whereU_(o) and u are reference voltages corresponding to an initial voltageand a final voltage and U_(B) the pole voltage of the battery suppliedby way of the measuring conductor for pole voltage, one of saidcomparators giving a level shifted output and the other of saidcomparators giving a level shifted output at the voltage u forrecharging.
 5. Apparatus according to claim 4 wherein a control unitcontains memory and time generating means which is response to the levelshifted outputs of the comparators shifts between first and secondrecharging levels are effected.
 6. Method according to claim 2 whereinsupplied charge is measured and the first and second recharging step isfollowed by a certain time interval disconnected without chargingcurrent (T_(pause)) contained in a charging cycle and carrying out anumber of cycles until a state of full charge corresponding to a certainamount of charge (Q_(max)) is reached.
 7. In an apparatus for high-ratecharging of a battery having a control means comprising an input, outputand a control electrode, said input being connected in series with a DCcurrent source and the output of the control means being connectable toa battery for supplying a charging current thereto, the improvementcomprising:a) a source of first voltage reference (U_(o)); b) a sourceof second voltage reference (u); c) a first summing circuit comprisingfirst and second inputs and an output supplying a first differentialcontrol voltage (U_(B) -U^(o)) from the first voltage reference and thebattery pole voltage; d) a second summing circuit comprising first andsecond inputs and an output supplying a second differential controlvoltage (u-U_(B)) from the second voltage reference and the battery polevoltage; e) switch means comprising first and second terminals, anoutput terminal and a control means, said first and second terminalsbeing connected to respective outputs of said first and second summingcircuits; f) a control circuit containing g) an amplifier connected tothe control electrode of the control means to control the chargingcurrent, an input of said amplifier being connected to the outputterminal of said switch means; h) a first voltage comparator having afirst input, a second input and an output, said first input of the firstvoltage comparator being connected to the first reference voltage; i) asecond voltage comparator having a first input, a second input and anoutput, said first input of the second voltage comparator beingconnected to the second reference voltage; j) a battery pole voltagesense line connected to the second inputs of each of said voltagecomparators for measuring the battery pole voltage; k) a control unitcomprising first and second inputs and first and second outputs, thecontrol unit including memory and timing circuits to control and measurethe energy supplied to the battery, said first input of said controlunit receiving the output signal of the first comparator, which signalterminates the first step of a recharging cycle and activates the firstoutput connected to the control means of the switch means, the controlmeans thereby shifting the switch to connect the input of the amplifierto the output of the second summing circuit to complete a rechargingcycle in a second step; and l) discharging means controlled by thecontrol unit to obtain a correct initial reference level for charging,the reference level being referred to as approximately zero charge.